1. Field of the Invention
The present invention relates to a method of manufacturing a disk-substrate used for a magnetic recording medium, the disk-substrate for the magnetic recording media, a method of manufacturing the magnetic recording media, the magnetic recording media, and a magnetic recording device.
2. Description of the Related Art
Recently, a hard disk drive (HDD) is often used as a storage device in a variety of devices such as personal computers and digital home appliances.
A HDD essentially comprises a disk-shaped magnetic recording media (hereinafter referred to as a “magnetic disk”) which is called a hard disk (HD), a spindle motor for rotating a magnetic recording media, an arm having a magnetic head attached thereto, and a servomechanism controlling the arm. The magnetic head is used to write magnetic information on the magnetic disk and read the magnetic information from the magnetic disk.
Recording techniques used for the HDD include longitudinal magnetic recording and perpendicular magnetic recording. The longitudinal magnetic recording aligns data bits horizontally in the circumferential direction on the surface of the magnetic recording medium disk. In contrast, the perpendicular magnetic recording aligns data bits perpendicularly to the surface of the magnetic recording medium disk. Recently, in response to the request to increase the recording density, the perpendicular magnetic recording is coming into practical use. However, the longitudinal magnetic recording, which has conventionally been in practical use and has a proven track record over the years, is still in vogue.
Currently, in the process of fabricating a magnetic disk generally used for a longitudinal magnetic recording HDD, onto a non-magnetic disk substrate are deposited a non-magnetic metal underlayer formed of a Cr film and the like, a magnetic recording layer formed of a Co alloy magnetic film and the like, a protective layer formed of an amorphous carbon film and the like, and a lubricant layer formed of a liquid lubricant. The non-magnetic metal underlayer, the magnetic recording layer and the protective layer can be formed by sputtering and the lubricant layer can be formed by coating. Each of these layers may comprise a plurality of layers as necessary. A variety of layers may be interposed between the above layers for various purposes.
In the HDD, when the magnetic information is read/written, the magnetic head is out of contact with the magnetic disk. Because of the rotation of the magnetic disk, the magnetic head is slightly flown.
If the surface of the magnetic disk is completely flat, the magnetic head cannot be stably flown even when the magnetic disk is rotated, due to, for example, the adhesion between the magnetic disk and the magnetic head. Typically, asperities are uniformly distributed on the surface of the magnetic disk for the purpose of achieving the flying stability of the magnetic head.
In a typical method of providing asperities on the surface of the magnetic disk, asperities are first formed on the surface of the disk substrate. Then, the non-magnetic metal underlayer, magnetic recording layer, protective layer, lubricant layer and the like are deposited on the disk substrate with the asperities. As a result, asperities corresponding to the asperities on the disk substrate are provided on the magnetic disk surface.
If the asperities on the magnetic disk surface have a large size, this causes an increase in distance between the magnetic disk and the magnetic head (flying height). In recent years, with an increase in the recording density of a HDD, the flying stability thus required becomes shorter and shorter. To meet the requirement of shorter flying stability, the magnetic disk surface is required to have uniform and nanometer scale asperities.
A method called texturing is known as a typical process for providing asperities on the disk substrate surface. In texturing, typically, a polishing tape is sent bit by bit to and pressed against the surface of the disk substrate which is rotated in the circumferential direction, to form approximately concentric line traces.
Ways of accomplishing such texturing include “fixed abrasive” texturing that uses a polishing tape with abrasive grain existing on its surface, and “free abrasive” texturing that uses a polishing tape without abrasive particles. In the case of using the polishing tape without abrasive particles, the texturing is performed while slurry including abrasive grain is applied as free abrasive grain.
In the “free abrasive” texturing, the processing speed of texturing, the precision of texturing, the shape of asperities thus formed, the line width of the line trace, the surface roughness, the uniformity of the asperities, and the like are greatly varied depending upon the type of polishing tape or slurry being used. Also, since such polishing tapes and slurry are consumable articles, the durability is of importance in terms of costs for practical use. In consequence, a choice and a combination of the polishing tapes and slurry are of critical importance.
Various types of polishing tapes used in the “free abrasive” texturing are proposed. Among others, recently, attempt has been made to reduce the diameter of fiber used in the polishing tape, in order to achieve minute asperities. The use of fiber having a fiber diameter of micrometer order, called microfiber, and the use of fiber having an ultrafine fiber diameter of nanometer order, called nanofiber, are proposed. For example, Japanese Patent Application Laid-Open No. 2002-79472 discloses an abrasive sheet for texturing a magnetic recording medium, which comprises ultrafine fiber having a fineness of 0.03 dtex (decidex) or less. Fineness (tex) represents fiber diameter. 1 tex is 1 g/1000 m length. Fineness is a function of a specific gravity and the diameter when the fiber is perfectly circular in cross section. Specifically, 0.03 dtex represents a diameter of 1.8 μm in the case of Nylon 6 having a specific gravity of 1.14, and represents a diameter of 1.7 μm in the case of PET having a specific gravity of 1.39.
Japanese Patent Application Laid-Open No. 2002-172555 discloses abrasive base fabric including microfiber with an average diameter of from 0.3 μm to 10 μm located in its central portion and including microfiber with an average diameter of from 0.05 μm to 1 μm located in its periphery portion, and describes that the base fabric is suitable for use in the texturing carried out in the process of manufacturing a magnetic recording substrate for a hard disk or the like.
Japanese Patent Application Laid-Opens No. 2003-170347 and No. 2003-170348 disclose abrasive base fabric including microfiber with an average diameter of 2 μm or less and abrasive base fabric including microfiber with an average diameter of from 0.05 μm to 2 μm, and describes that the base fabric is suitable for use in the texturing carried out in the process of manufacturing a magnetic recording substrate for a hard disk or the like.
Japanese Patent Application Laid-Open No. 2005-329534 discloses abrasive fabric having nanofiber with a number averaged monofilament fineness ranging from 1×10−8 dtex to 4×10−4 dtex, and describes that the abrasive fabric is used in texturing a substrate provided for manufacturing a magnetic recording medium. A monofilament fineness of from 1×10−8 dtex to 4×10−4 dtex represents a monofilament diameter of from 1 nm to 200 nm in the case of Nylon 6 (specific gravity 1.14 g/cm3).
The slurry used in the “free abrasive” texturing comprises abrasive of diamond abrasive grain, alumina abrasive grain or the like dispersed in a solvent such as water. As necessary, an additive such as a dispersant or a surfactant may be added to the slurry. The material, size and shape of the abrasive greatly affect the texturing process time and a texture shape. Various types of grains are proposed as a diamond abrasive grain generally used in the texturing. For example, Japanese Patent Application Laid-Open No. H08-7264 discloses a texturing process in grinding by monocrystalline diamond slurry having a particle diameter of 3 μm or less. Japanese Patent Application Laid-Open No. H11-138424 discloses a slurry fluid for texturing the surface of a magnetic hard disk substrate, which contains polycrystalline diamond fine-powder with an average particle diameter d50 of from 0.05 μm to 5 μm. Japanese Patent Application Laid-Open No. 2002-030275 discloses a texturing fluid for texturing a magnetic disk substrate. The texturing fluid includes agglomerated polycrystalline diamond particles of an average particle diameter of from 0.01 μm to 2 μm. Each of the agglomerated polycrystal diamond particles consists of polycrystalline diamond particles of an average particle diameter of from 1 nm to 20 nm.
Japanese Patent Application Laid-Opens No. 2004-178777 and No. 2004-259417 disclose abrasive slurry used for texturing the surface of a magnetic hard disk substrate. The abrasive slurry includes monocrystalline diamond particles, polycrystalline diamond particles or cluster particles comprising the monocrystalline and polycrystalline diamond particles, these particles having a particle diameter ranging from 1 nm to 10 nm or from 1 nm to 50 nm. Japanese Patent Application Laid-Open No. 2000-136376 discloses abrasive particles which are non-cohesive aggregates of fine monocrystalline particles of diamond which have an average particle diameter of 5 μm or less. The surface of the diamond particles is covered with non-diamond carbon. The abrasive particles are used for texturing a nickel-coated aluminum substrate which is used for producing a hard disk. Japanese Patent Application Laid-Open No. 2005-131711 discloses diamond abrasive particles made of artificial diamond produced through an impact method, in which the average diameter of the secondary particles ranges from 30 nm to 500 nm and the diameter of the primary particles is 20 nm or less. The abrasive particles are suitable for use in abrading or texturing a magnetic hard disk substrate. Also, International Publication WO2006/006721 discloses a composition for texturing including nano-diamond crystal cluster produced by detonation of an oxygen-deficient explosive, and describes that the composition is used for texturing an underlayer of an aluminum-made magnetic disk or the surface of a glass-made magnetic disk.
In addition to producing uniform asperities on the surface of a magnetic disk for the purpose of stably flying the magnetic head, the texturing also has a role in eliminating, from the disk substrate surface, scratches which lead to a magnetic-recording error and abnormal projections with which the magnetic head may come into collision to cause head crash, as described in the aforementioned documents and the like. In particular, in a longitudinal magnetic recording medium, the texturing has a role in unidirectionally aligning the crystal orientations of the particles included in the non-magnetic metal underlayer and the magnetic recording layer so as to contribute to the improvement of the recording density.
One of effective ways to improve the S/N ratio of the longitudinal magnetic recording medium is an increase in anisotropy between the magnetic properties in the circumferential direction and the radial direction (in particular, an OR−Mr·t ratio between residual magnetization-thickness products (Mr·t) in the circumferential direction and the radial direction). It is known that the texturing which forms grooves in the circumferential direction is useful for an increase in in-plane magnetic anisotropy.
There are some examination cases and are proposed some theories, as to the mechanism of an increase in in-plane magnetic anisotropy by the texturing.
For example, “Coercivity Orientation and Microstructure of CoCrPtTa Thin-Film Magnetic Recording Media” by Katsunori Takahashi et al. (Journal of Magnetics Society of Japan, 2000, vol. 24, pp 283-286), and “Study of In-Plane Magnetic Anisotropy of Thin-Film Media” by Reiko Murao et al. (Journal of Magnetics Society of Japan, 2001, vol. 25, pp 615-618), study magnetic recording media having a Cr alloy underlayer film, a CoCrPtTa alloy magnetic film, and a DLC protective film sequentially deposited, by DC magnetron sputtering, on the NiP/Al—Mg substrate which has been textured. According to those literatures, when the diameter of a crystal particle is smaller than the half width of a fine texture groove, the texture shape effects result in the occurrence of stress anisotropy in the circumferential direction and the radial direction on the substrate surface. In turn, the stress anisotropy produces in-plane distortion of the crystal lattice of the Cr underlayer film. When the Cr (110) interplanar spacing in the circumferential direction becomes smaller, an axis of easy magnetization of CoCrPtTa is preferentially oriented in the circumferential direction, resulting in an increase in in-plane magnetic anisotropy.